In U.S. Pat. No. 4,987,429 to R.B. Finley, G.F. Day and D.J. Devine there is a description of problems encountered in electrostatic printing of the kind employing liquid toners. There has been an evolution in electrostatic color printing technology from the use of multiple toner applicators, i.e. one per color, to a system in which a single shared applicator is employed, residual toner being cleaned from the applicator after each color pass. The benefits of this approach are that residual toner no longer dries out, which necessitates manual cleaning of the applicator, and the number of components in such a printing system is reduced. The patent to Finley et al. describes the problems which occur with a liquid toning system which uses a single applicator for multiple toner colors. Since several colored liquids share the same volume of tubing, piping, and applicator and the like, some color cross-mixing is inevitable and this results in poor imaging and the loss of color saturation as well as in premature toner disposal. Liquid toner disposal is costly for the user. The replacement cost is high for colored liquid toners and the expense for proper disposal of the spent liquids is escalating because of government agency regulations.
The patent to Finley et al. teaches an improvement in single applicator color printing by means of a single pump for all colors with a selector valve selectively connecting the pump input to one of the various colored toners, to wash fluid, or to room air. By the use of the "liquid" pump, with its input connected to room air, for the purpose of liquid purging with air, most of the liquid can be expelled prior to the introduction of the next selected liquid. This reduces the effective "common volume" which is shared by all the liquids and permits a single pump to be shared by all colors without excessive color cross contamination. The patent also teaches the use of a small toning applicator or shoe which scans a drum-supported sheet in a helical pattern. While the contribution of Finley et al. is significant, a problem which is inherent in helical scan systems is that of visible image banding in which the boundaries of the helical stripes are visible. For this reason, most electrostatic printers use a full-width toning applicator so that there are no toning boundaries within the image. Increasing demand for pictorial type imaging as opposed to line drawings is accelerating the need for highly uniform imaging and toning characteristics.
One might then attempt to adapt the one-pump approach of Finley et al. to full width applicator systems. For example, in large format web-based printers manufactured by Xerox Corporation, as described in U.S. Pat. No. 4,569,584 to St. John et al., an electrostatic printing system applies color toners successively by moving a web back and forth past multiple toner applicators, with the position of the web carefully controlled by optical registration marks along the edges of the web. Could the one-pump system of Finley et al. be adapted to the web system of St. John et al., with the multiple toning stations of St. John et al. replaced with a single full-width toning shoe? The answer is not clear because, even if this modification could be done, the purging system uses a toner pump with a characteristically low volume flow rate for liquids and this results in very slow moving purge air. Such slow-moving air is only partly effective in expelling liquids from the tubing, pipes and applicator and the like so that a significant volume of liquid residual remains even after lengthy air purging. While this may suffice for a small scanned toning shoe-based system, the full-width system of St. John et al. is many times larger and the residual liquid remaining even after air purging results in poor image quality and early toner disposal. What is needed is a more effective method of expelling the residual liquids from the common volume, including the liquid pump itself. The difficulty arises basically because of the extreme differences between the fluid properties of toner and air. Air is about 54 times less viscous and 630 times less dense than toner or wash fluid. For this reason, pumps which are effective at moving toner fluid are extremely ineffective at the task of moving air and vice-versa.
An object of the invention is to devise an electrostatic printing system which has the high-speed and image uniformity advantages of web-based systems while preserving multiple benefits of a single applicator, one-pump system.